Exhaust after-treatment devices, equipped on internal combustion engines to comply with emission regulations, have proven to be sensitive to the combustion by products of the fuel and lubricant used in the engine. In addition, certain types of devices are sensitive to one or more of the following: (1) phosphorus coming from the lubricant, (2) sulfur coming from both fuel and lubricant, and (3) sulfated ash resulting from the combustion of fuel and lubricant. In order to ensure the durability of the different types of after-treatment devices, special lubricants are being developed that feature relatively low levels of, for example, sulfur, phosphorus, and sulfated ash.
Several challenges exist when formulating an automotive engine lubricant that is essentially free of Zinc Dialkyldithiophosphate (ZnDTP). ZnDTP is a versatile, anti-wear/anti-oxidant component that provides good wear and favorable oxidation protection under severe conditions. However, ZnDTPs comprise the elements zinc, sulfur and phosphorus which all have negative impact on exhaust after-treatment devices.
To compensate for the loss in antiwear and antioxidancy from ZnDTP, molybdenum-containing lubricating oil compositions were developed to advantageously provide high wear inhibition when used in an internal combustion engine while containing relatively low levels of sulfated ash content. However, a problem that was encountered when using high levels of molybdenum to compensate for the loss of ZnDTP was copper corrosion performance of the lubricating oil.
There is a need solve the problems described above. Original Engine Manufacturers have required passing the ASTM D6594 Test (HTCBT) and ASTM D130 test (Copper Strip Corrosion Test) to qualify lubricating oils for use in their engines. The challenge for ZnDTP-free oils is to develop a lubricating oil composition which maintains the wear performance of a conventional automotive lubricating oil, while at the same time preventing corrosion, and also ensures durability of the different types of after-treatment devices. The present inventors have developed a solution to this problem.
The inventors have discovered that not just any copper corrosion inhibitor provides sufficient copper corrosion performance in a high molybdenum containing and essentially ZnDTP-free oil. A specific chemistry is required. In addition, this chemistry enables the high wear inhibition performance to be achieved with the molybdenum-containing lubricating oil compositions of the present invention while also employing relatively low levels (or substantially free) of any phosphorus and zinc content.